The invention relates to an x-ray tube with a static cathode and a rotating anode core that preferably has a conical target and is located inside the stationary anode.
In stationary-anode x-ray tubes according to the prior art (e.g., LOHMANN Type 160/25 HA), the maximum achievable thermal load of the focal spot is limited by the target material. Particularly with small focal spots and high tube current, as is required for high-resolution imaging methods, the target material is eroded during operation, before effective cooling of the target is possible. As a result of the increasing erosion of the target material, the operating values of a conventional stationary-anode tube of this kind deteriorate, which can ultimately lead to its complete failure. Consequently, tubes of this kind are primarily used for low focal spot loads, when sensitive detectors are available or when long exposure times are acceptable.
By rotating the target under the static focal spot, this disadvantage is prevented in that the area subject to thermal load is enlarged in annular fashion. In rotating-anode x-ray tubes of this kind (e.g., VARIAN Type A145), an electron beam is focussed well outside the longitudinal axis of the tube on an anode disk of relatively large diameter. This known structural design is capable of realizing high currents with relatively small focal spots. The thermal load is compensated for by the thermal capacity of the anode disk when the tube is used in pulse operation. The relatively rapid rotation of the heavy anode disk necessitates a complex bearing mount and can still cause undesirable vibrations that impair the imaging quality. Moreover, it is difficult to cool the anode directly. Thus, rotating-anode x-ray tubes are relatively complex, large and expensive.
An object of the invention is to design an x-ray tube which, while providing good imaging quality, permits a greater load on the focal spot compared to stationary-anode tubes and is also more compact, of simpler design and thus cheaper than rotating-anode tubes.
This object is solved by an x-ray tube according to the invention with a stationary cathode and an anode, wherein the anode comprises a stationary shell and an essentially cylindrical core pivot-mounted inside the shell that has a target on the end facing the cathode, and wherein the shell has openings outside the center axis on the end facing the cathode and on the side surface at the height of the target to allow passage of the accelerated electrons and the x-rays generated in the target. Preferred configurations are presented below and in the dependent claims.
The shell can essentially be designed as a hollow cylinder and encloses the likewise essentially cylindrical core. The end of the shell facing the cathode is provided with an opening outside the longitudinal axis of this arrangement, which permits the electrons emitted by the cathode and accelerated by the electric field to pass through to the core. However, the interior of the shell and the core can also be of other shapes that match one another and permit rotation of the core, such as nesting, truncated cones.
The side surface of the shell has another opening at the height of the target, which permits the x-rays generated in the target to pass to the outside.
The preferred materials for the shell are heavy metals, such as copper and tungsten, as well as alloys and composite materials made of these metals (copper for the core and tungsten and tungsten-rhenium for the target).
In a preferred configuration, the anode core is mounted only at the two ends. Furthermore, the bearings are preferably mounted in the center axis and designed as toe bearings. In this context, one toe rests on a preferably concave surface. The material of the toe and the surface can be the same or different and preferably has a medium to high hardness. Suitable materials are, for example, steel, diamond, ceramics, tungsten.
The target mounted on the end of the core facing the cathode is advantageously in the shape of a cone, which preferably has an aperture angle of 0 to 45 degrees. In this case, the toe bearing can be mounted on the cone tip. The target can also be in the form of a flat, truncated cone or a disk.
The anode is preferably designed such that it is possible for the core to achieve rotational speeds of between 1 and 1000 rpm.
The gap between the inside surface of the shell and the core is preferably no more than 1 mm, a gap of 0.1 to 0.5 mm being particularly preferable.
As the interior of the shell is connected to the tube vacuum, it must be sealed off from the atmosphere. This is achieved by means of a wall, preferably made of vacuum-tight, non-magnetic material, such as copper, stainless steel or beryllium. The wall can be connected to the shell by known methods, such as soldering or welding.
Means for driving the pivot-mounted core inside the tube can be provided outside the interior of the tube, preferably on the atmosphere side of this wall. These means can be a moving magnetic field, if the core is magnetic or magnetizable. A moving magnetic field can be generated by one or more rotating magnets. The magnet can at least partially be designed as an impeller, or built into one, and be located in a cavity through which a fluid (e.g., oil, air, water) flows, so that it is driven by this flow. Another preferred drive is a motor, e.g. an electric motor.
For example, two inserts that are preferably made of iron can be built into the end of the rotating core and a pair of magnets into an impeller, which is separated from the vacuum space by a thin metal wall and guided in its own, small chamber that is open towards the generator housing. For the oil-flow drive, insulating oil is pumped from the generator housing via a hose connection into this chamber outside the vacuum space, thus driving the impeller. This causes the inside anode core to rotate and, at the same time, heat from the end of the anode is transported with the oil flow.
A fluid, preferably oil, can also flow through the shell for cooling, and the shell can be provided with suitable bore holes and hoses for this purpose.
The shell enclosing the rotating core also acts as an anode housing with window for improving smooth electrical running. In this case, it is preferably made of heavy metal, in order to avoid undesirable back reflection. In addition, the window only permits radiation to emerge in the desired direction and thus serves the purpose of radiation protection. A special anode housing, such as described for a rotating-anode tube in U.S. Pat. No. 4,309,637, is not necessary.
This arrangement permits very precise guidance of the target and, ultimately, sharply defined focal spot imaging.
Another advantage of arranging the rotating core inside the shell is the possibility of conducting heat out of the core and into the shell by means of thermal radiation. This is made possible by a narrow gap between the core and the shell and can also be further promoted by dulling, blackening or roughening the opposing surfaces. The shell itself can be cooled simply and directly using a fluid, preferably oil.
Furthermore, this arrangement permits simple magnetic coupling of the rotating anode core with magnets installed outside the vacuum space, these in turn being easily driven by an electric motor or an oil flow, for example. For this purpose, two inserts that are preferably made of iron are built into the end of the rotating core and one or more magnets into an impeller, for example, which is separated from the vacuum space by a thin metal wall and guided in its own, small chamber that is open towards the generator housing.
Overall, the strictly axial arrangement of the anode core with anode housing and the 2-toe bearing, that is possible as a result, ensure a slim and inexpensive design.
The tube according to the invention can be installed in a high-voltage generator without requiring a separate tube housing. It can be used for imaging procedures, e.g., in medical diagnostics and material testing.